Problem: 6 people can paint 3 walls in 50 minutes. How many minutes will it take for 10 people to paint 10 walls? Round to the nearest minute.
Solution: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 3\text{ walls}\\ p &= 6\text{ people}\\ t &= 50\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{3}{50 \cdot 6} = \dfrac{1}{100}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 10 walls with 10 people. $t = \dfrac{w}{r \cdot p} = \dfrac{10}{\dfrac{1}{100} \cdot 10} = \dfrac{10}{\dfrac{1}{10}} = 100\text{ minutes}$